In the past few years, there has been considerable interest exhibited in surface wave type acoustooptic interaction for use as Bragg modulators and deflectors, for mode converters, for fast switches, and the like.
The interaction of guided optical waves and surface acoustic waves in only one direction leads to better interaction efficiency than has been obtainable in bulk wave counterparts. For example, a diffraction efficiency of 85% and a -3 dB bandwidth of 100 MHz, using only 190 mW of drive power has been accomplished using a surface acoustic wave frequency of 180 MHz.
Although diffraction efficiency is very important, bandwidth is the most important characteristic in many important applications, and wide bandwidth has been lacking in single transducer acoustooptic devices. For example, wide bandwidth provides a larger number of scannable spot diameters and a higher switching speed of the deflected light beam. However, in the past where a single transducer design was developed to provide both high diffraction efficiency and wide bandwidth, large RF driving power was required, which power can easily result in failure of the interdigital transducer.
More recently, experiments have been conducted using multiple transducer configurations in what are known as tilted, and phased surface acoustic wave devices. In the tilted system, the interdigital transducers have staggered center frequencies and their propagation axes are tilted with respect to each other. The surface acoustic waves produced by this type of transducer array satisfy the Bragg condition in each frequency band, and thus can provide a broad composite frequency response. In the phased system, all of the transducers have the same center frequency and propagation axis, but the transducers are arranged in a stepped height configuration to introduce a phase shift between adjacent acoustic waves. This allows the resultant wavefront to be scanned by varying the acoustic frequency. Efficient diffraction is then possible over a relatively wide frequency band because scanning of the wavefront enables the composite acoustic beam of large aperture to track the Bragg condition.
A rather thorough description of the single and multiple transducer systems of the prior art may be obtained by viewing such articles as "Wide-Band Guided-Wave Acoustooptic Bragg Diffraction and Devices Using Multiple Tilted Surface Acoustic Waves," by Chen S. Tsai, M. A. Alhaider, Le Trong Nguyen, and Bumman Kim, in the Proceedings of the IEEE, Vol. 64, No. 3, March 1976, and "Thin-Film Acoustooptic Devices" by Eric G. H. Lean, James M. White, and Christopher D. W. Wilkinson, in the Proceedings of the IEEE, Vol. 64, No. 5, May 1976.
In contrast to the techniques used in the prior art to provide both high diffraction efficiency and broad bandwidth, the present technique provides a continuous variation of optical beam angle with frequency in a single transducer configuration to eliminate the problems associated with driving multiple discrete transducers.